The Medrash states that Avraham asked Yitzchak what he saw and he answer that he saw the divine presence. However,when he asked Eliezer and Yishmael they both answered that they saw nothing. The passuk says that Avraham left them behind with the donkey. My question is,it is understandable that he left both Eliezer and Yishmael behind, but why leave the donkey which would be helpful carrying the wood for the alter?